From: "J. Bruce Fields" Subject: Re: [PATCH] ext4: turn on i_version updates by default Date: Mon, 14 May 2012 14:54:00 -0400 Message-ID: <20120514185400.GA32026@fieldses.org> References: <20120514140618.GA29902@fieldses.org> <9124E59E-2479-4C32-A528-3237B48DEC01@dilger.ca> <20120514152334.GB29902@fieldses.org> <14B38D68-FAE4-444A-BCD9-7EBF7E1BBFE1@dilger.ca> <20120514175822.GC1439@thunk.org> <20120514183316.GA1894@localhost.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Ted Ts'o , Andreas Dilger , "linux-ext4-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , "linux-nfs-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , "linux-fsdevel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" To: Josef Bacik Return-path: Content-Disposition: inline In-Reply-To: <20120514183316.GA1894-bi+AKbBUZKY6gyzm1THtWbp2dZbC/Bob@public.gmane.org> Sender: linux-nfs-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: linux-ext4.vger.kernel.org On Mon, May 14, 2012 at 02:33:17PM -0400, Josef Bacik wrote: > On Mon, May 14, 2012 at 01:58:22PM -0400, Ted Ts'o wrote: > > On Mon, May 14, 2012 at 11:27:42AM -0600, Andreas Dilger wrote: > > > > And if it at all possible I'd rather have it be something that Just > > > > Works rather than something that requires extra configuration. > > > > > > Sure, but this is only useful for NFSv4, but costs everyone using > > > ext4 continuous overhead, so it isn't a clear-cut case to enable > > > the version just on the thought that NFS might one day be used on > > > any particular filesystem. > > > > It's not a matter of "NFSv4 might one day be used"; if we don't turn > > on i_version updates until the file system is actually exported via > > NFSv4, there would be no deleterious effects. > > > > I always thought that was going to be the plan; that there would be > > some flag that would be set in struct super_block when the file system > > was exported that would enable i_version updates. > > > > That way we satisfy the "no extra configuration" needed requirement, > > which I agree is ideal, but we also don't waste any CPU overhead if > > the file system is not exported via NFSv4. I tried to implement > > anything along these lines because I don't care enough, and I don't > > use NFSv4 personally.... > > > > Seems like this is just a bad place to be doing inode_inc_iversion(). If > MS_IVERSION is set we will update iversion in file_update_time() and then call > mark_inode_dirty which will jack up the iversion again. Agreed, that's weird. > In btrfs we just change > it wherever we change ctime and that way you don't really notice the extra > overhead since you are doing it in paths where you are changing a bunch of stuff > in the inode already, and mostly where you hold the i_mutex so you aren't going > to be hitting any contention on the i_lock. Thanks, I don't think they're worried about the inode_inc_iversion() calls themselves, but the behavior of file_update_time(): if (!timespec_equal(&inode->i_mtime, &now)) sync_it = S_MTIME; if (!timespec_equal(&inode->i_ctime, &now)) sync_it |= S_CTIME; if (IS_I_VERSION(inode)) sync_it |= S_VERSION; if (!sync_it) return; ... mark_inode_dirty_sync(inode); So now mark_inode_dirty_sync() is called on every update, instead of merely on every update that sees a time change (so at most once a jiffy). So mark_inode_dirty_sync (and hence ->dirty_inode = ext4_dirty_inode) may get called more often if you're writing very frequently. I'm a bit surprised that's expected to add significant overhead to the write. I guess I should stare at the code and try to follow Andreas's explanation.... --b. -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org More majordomo info at http://vger.kernel.org/majordomo-info.html